This invention relates to a method of repairing a wall having two or more layers. More particularly, this invention relates to a method of repairing a wall of chemical process equipment wherein one of the layers has tantalum as a base constituent.
Tantalum and its alloys are commonly used in environments in which their superior corrosion resistance can be relied upon to provide long lived components which are exposed to corrosive solutions such as sulfuric acid, hydrochloric acid, and nitric acid. Thus, tantalum and tantalum alloys are used in chemical process equipment and are frequently used to line vessels containing corrosive solutions. For example, Fansteel "63" Metal, manufactured by Fansteel, Inc., has tantalum as a base constituent and has 2.5 weight per cent tungsten and 0.15 weight per cent columbium, and has high corrosion resistance and excellent strength properties at 390.degree. F. (200.degree. C.) which is a maximum temperature requirement for a number of chemical process industry applications. Its fabricating and welding characteristics are similar to those of commercially pure tantalum.
The outstanding corrosion resistance and inertness of tantalum and its alloys is due to the thin surface film of tantalum pentoxide. When this film is chemically damaged, the underlying metal is attacked. Thus, faults such as corrosion pits may occur in liners having tantalum as a base constituent.
Heretofore, repair techniques for liners having tantalum as a base constituent have not consistently resulted in high quality, contamination-free welds. It has been attempted to remove areas of the liner where corrosion pits have perforated the liner and to weld tantalum patches over these areas. However, severe contamination of the weld areas frequently occurs, particularly underneath the liner, because of residual corrosion products.
As is known, at temperatures in excess of about 400.degree. C., it is necessary to protect tantalum and its alloys against contamination and its tendency to react with gases, vapors and other metals. The high melting point (3,000.degree. C.) of tantalum and its alloys, (such as Fansteel "63" Metal), together with the strong tendency of hot solid or molten tantalum to alloy with, or be contaminated by, other metals, solids and gases, makes it difficult to weld tantalum or its alloys without contamination.